With work nearly completed on a new material to shield astronauts from radiation, saboteurs prepare for one final assault. Complicated inter-personal relationships within the team suggest a traitor and Saunders, from Security, tracks the gang. Meanwhile, Steed tracks Saunders and rescues him from being killed. With the saboteurs in custody, the traitor is unmasked as Susan Summers, Alford's assistant, and Steed neutralises this final threat.